The interaction between oxytocin and neurophysin is being studied via 13C nuclear magnetic resonance spectroscopy. To this end, several oxytocin derivatives enriched with 13C (90%) in various positions have been synthesized. In the presence of neurophysin, the peak due to the enriched carbon stands out above the background in the 13C NMR spectrum. By comparing results obtained for different enriched positions, a detailed understanding of oxytocin-neurophysin interaction at the atomic level may be obtained. Useful information has been obtained by varying the concentration, temperature, and pH at which the experiments have been conducted. Also relaxation time (T1 and T2) measurements are being used to determine motional parameters.